A golden block, which weighs a hundred kilos, was stolen from a museum in Central Japan on Wednesday.
The valuable exhibit was not protected by sensors as the museum's management board decided that the visitors should be allowed to touch it.
For the same reason, all the other windows were also open.
Three disguised men took advantage of the strange situation.
The robbers entered the museum from a natural cave, located in its back, police officials reported.
After stealing the gold, the criminals were taken by a car, driven from an accessory of theirs.
The case is still under investigation.